Glass fibres can be produced by melting a glass composition in a melter and drawing them out through the tip plate of a bushing assembly. A bushing assembly is generally in the form of a rectangular box with two pairs of opposed side and end walls forming the perimeter thereof with an opening at the top in fluid communication with the melter. The bottom floor comprises a tip plate which is a plate comprising a multitude of orifices or tips through which the glass melt can flow to form fibres, which are attenuated to reach their desired diameter. To ensure an optimal control of the viscosity, the temperature of the tip plate must be controlled accurately. The temperature of the tip plate depends on the glass melt composition but is generally well above 1200° C. Because of the extreme working conditions, the various components of a bushing assembly are made of platinum or platinum alloys, typically rhodium-platinum alloys.
The bushing is heated electrically by passing current through the body of the bushing from a first connector clamped to a first terminal ear, thermally coupled to a first end wall of the bushing body, to a second connector clamped to a second terminal ear, thermally coupled to a second end wall of the bushing body, opposite the first end wall. To prevent the connectors, usually made of copper, from overheating and from deforming, they are usually water cooled. The terminal ears are therefore exposed to severe thermal gradients, between the free end thereof, where water cooled connectors are clamped and the end coupled to the bushing end wall which is at temperatures of well above 1200° C. Such extreme temperature gradients have two drawbacks; first they create substantial strain in the terminal ears which are fixed at both opposite ends of the bushing body because of varying levels of thermal expansion as the temperature varies, leading to warpage of the ears. Second, the cooling of the connectors also cools the end walls of the bushing, when a homogeneous temperature is required at the level of the bushing tip plate. To solve the latter problem, US2003/0167802 proposed to provide at least one V-shaped notch at or near an unattached end of the ear. A similar design is proposed in FIG. 6 of U.S. 6,196,029. The problem is that, even though the cross sectional area of the unattached end of a terminal ear which is being cooled compared with the end coupled to the bushing end wall is smaller than in the case of an un-notched terminal ear, the density of current passing through the electrodes required to heat the bushing is higher and the electrodes therefore require more cooling, so that what is gained on the one hand by reducing the cooled portion of the terminal ear is wasted on the other hand by requiring more cooling to maintain the smaller electrodes at their working temperature. Furthermore, internal stresses tend to concentrate at the tip of the V-notch, leading to the premature failure of the terminal ear, and thus to the interruption of the production.
Terminal ears with I-shaped notches have been proposed in the art, such as in CN2516548U and in US2006/0218972. In the latter document such notches are meant to receive a bolt to fix an electrical connector to a terminal ear. The design of the terminal ear disclosed in CN2516548U is the equivalent of the one disclosed in US2003/0167802 discussed above, having an I-shaped notch instead of a V-shaped notch. The connectors size is larger in an I-shaped notch, but stress concentration at the tip of the notch is still very high.
There remains a problem to allow the optimum design for reducing cooling of the bushing end walls by the connectors cooling systems, and concomitant reduction of stress concentration in the terminal ears, leading to their premature failure. This and other problems are solved by the present invention.